Modified oligonucleotides are of great value in molecular biological research and in applications such as anti-viral therapy. Modified oligonucleotides which can block RNA translation, and are nuclease resistant, are useful as antisense reagents. Sulfurized oligonucleotides, which contain phosphorothioate (P—S) linkages, are of interest in these areas. Phosphorothioate-containing oligonucleotides are also useful in determining the stereochemical pathways of certain enzymes which recognize nucleic acids.
Standard techniques for sulfurization of phosphorous-containing compounds have been applied to the synthesis of sulfurized oligonucleotides. Examples of sulfurization reagents which have been used to synthesize oligonucleotides containing phosphorothioate bonds include elemental sulfur, dibenzoyl tetrasulfide, 3-H-1,2-benzidithiol-3-one 1,1-dioxide (also known as Beaucage reagent), tetraethylthiuram disulfide (TETD), and bis(O,O-diisopropoxy phosphinothioyl) disulfide (known as Stec reagent). Most of the known sulfurization reagents, however, have one or more significant disadvantages.
Elemental sulfur presents problems and is not suitable for automation because of its insolubility in most organic solvents. Furthermore, carbon disulfide, a preferred source of sulfur, has undesirable volatility and an undesirably low flash point. Unwanted side products are often observed with the use of dibenzoyl tetrasulfide. Beaucage reagent, while a relatively efficient sulfurization reagent, is difficult to synthesize and not particularly stable. Furthermore, use of Beaucage reagent forms a secondary reaction product which is a potent oxidizing agent. (R. P. Iyer et al., J. Am. Chem. Soc. 112, pp. 1253-1254 (1990); R. P. Iyer et al., J. Org. Chem. 55, 4693-4699 (1990)). This can further lead to unwanted side products which can be difficult to separate from the desired reaction product. Tetraethylthiuram disulfide, while relatively inexpensive and stable, has a sulfurization reaction rate which can be undesirable slow.
A method for producing a phosphorothioate ester by reaction of a phosphite ester with an acyl disulfide is disclosed in Dutch patent application No. 8902521. The disclosed method is applied to a purified phosphotriester dimer utilizing solution phase chemistry. The method is time and labor intensive in that it was only shown to work in a complex scheme which involved carrying out the first stage of synthesis (formation of a phosphite) in acetonitrile, removing the acetonitrile, purifying the intermediate phosphotriester, and proceeding with the sulfurization in a solvent mixture of dichloroethane (DCE) and 2,4,6-collidine. Furthermore, the method was demonstrated only with a dinucleotide. There was no suggestion that the Dutch method could be employed with larger nucleic acid structures, that the same could employ a common solvent throughout all steps of synthesis, that improved yields could be obtained, or that the method could be adapted for conventional automated synthesis without extensive modification of the scheme of automation. Although acetonitrile is mentioned as one of several possible solvents, utility of the method for carrying out all steps of the synthesis in acetonitrile as a common solvent was not demonstrated. While other publications (Kamer et al., Tetrahedron Letters 30(48), pp. 6757-6760 (1989); Roelen et al., Rech. Trav. Chim. Pays-Bas 110, pp. 325-331 (1991)) show sulfurization of oligomers having up to 6 nucleotides, the foregoing shortcomings are not overcome by the methods disclosed in these references.
Thus, there remains a need for improved methods and reagents for preparing sulfur-containing phosphorous groups, such as phosphorothioate linkages, in oligonucleotides and other organic compounds. The present invention is directed to these, as well as other, important ends.